Glaucoma is a disease of the eye characterized by increased pressure within the eyeball. If left untreated, glaucoma leads to a gradual impairment of sight which may result in blindness. The disease is prevalent among the population over 40 years of age, but develops slowly and often remains asymptomatic until well advanced.
Tonometry is the noninvasive measurement of intraocular pressure of the eye. Use of a tonometer for measurement of the intraocullar pressure is an important tool for the detection and treatment of glaucoma and other eye disorders. Typically, a tonometer includes a ring-and-plunger arrangement which is brought into contact with the eye. The plunger is mounted to permit movement relative to the ring. The force on the plunger with respect to the ring is then directly translated into intraocular pressure measurements.
To avoid pressure understatements or overstatements, it is imperative that the axis of the ring-and-plunger arrangement contact the eye perpendicular to the center line of the eye. The requirement of perpendicularity may be satisfied by use of an examination chair tonometer unit, but the cost of such units limits their effective availability. Methods of detecting perpendicularity of a tonometer with respect to an eye are utilized to safeguard against understatements of intraocular pressure. For example, U.S. Pat. No. 3,338,089 to Coombs, Jr. et al. utilizes photosensing for imaging of the area of contact with the eye under test, while U.S. Pat. No. 4,621,644 to Eilers teaches developing an area-proportional signal responsive to the area of contact.
Another source of error in intraocular pressure measurements stems from the variance of positioning force of the tonometer on the eye. With skilled handling this bias may be moderated, but an overstatement of 2 to 15 mm Hg may still occur depending upon the tonometer used. This is a significant problem since the clinical range of intraocular pressure for the population is between 8 to 28 mm Hg. To combat this problem U.S. Pat. No. 3,948,248 to Zuckerman et al. teaches using ultrasonic energy to observe ocular pulses. However, the ultrasonic measurement requires highly specialized equipment and a trained operator.
Other factors which must be considered in designing a tonometer that contacts an eye include the potential for cross contamination of patients. The bodily fluids of an eye may contaminate the eye of a subsequent user if a tonometer is not adequately sterilized after each use.
Conventional noninvasive measuring of intraoculator pressure is performed by contacting the cornea. Because of the sensitivity of the cornea, an anesthetic is applied injury is greater about the cornea than the potential of injury involved with contacting the sclera area of the eye. Measurements of intraocular pressure at the sclera area of the eye would eliminate the need of anesthetic administration and would open the field of tonometry to general practitioners and even to home use. But low-cost tonometers have as of yet been unsuccessful in gathering accurate intraocular pressure measurements at the sclera because of the tough shell of the sclera.
It is an object of the present invention to provide a low-cost tonometer which is accurate at the cornea and sclera, yet which does not require a highly trained operator.